/// <summary>
/// <code>
/// function FOL-BC-ASK(KB, goals, theta) returns a set of substitutions
/// input: KB, a knowledge base
/// goals, a list of conjuncts forming a query (theta already applied)
/// theta, the current substitution, initially the empty substitution {}
/// </code>
/// </summary>
/// <param name="KB"></param>
/// <param name="ansHandler"></param>
/// <param name="goals"></param>
/// <param name="theta"></param>
/// <returns></returns>
private IList <IList <ProofStepBwChGoal> > Folbcask(FOLKnowledgeBase KB,
BCAskAnswerHandler ansHandler, IList <Literal> goals,
IDictionary <Variable, ITerm> theta)
{
var thisLevelProofSteps = new List <IList <ProofStepBwChGoal> >();
// local variables: answers, a set of substitutions, initially empty
// if goals is empty then return {theta}
if (goals.Count == 0)
{
thisLevelProofSteps.Add(new List <ProofStepBwChGoal>());
return(thisLevelProofSteps);
}
// qDelta <- SUBST(theta, FIRST(goals))
Literal qDelta = KB.Subst(theta, goals[0]);
// for each sentence r in KB where
// STANDARDIZE-APART(r) = (p1 ^ ... ^ pn => q)
foreach (Clause r in KB.GetAllDefiniteClauses())
{
var standardizedR = KB.StandardizeApart(r);
// and thetaDelta <- UNIFY(q, qDelta) succeeds
IDictionary <Variable, ITerm> thetaDelta = KB.Unify(standardizedR.GetPositiveLiterals()[0].AtomicSentence, qDelta.AtomicSentence);
if (null != thetaDelta)
{
// new_goals <- [p1,...,pn|REST(goals)]
var newGoals = new List <Literal>(standardizedR.GetNegativeLiterals());
newGoals.AddRange(goals.Skip(1));
// answers <- FOL-BC-ASK(KB, new_goals, COMPOSE(thetaDelta,
// theta)) U answers
IDictionary <Variable, ITerm> composed = this.Compose(KB, thetaDelta, theta);
IList <IList <ProofStepBwChGoal> > lowerLevelProofSteps = this.Folbcask(
KB, ansHandler, newGoals, composed);
ansHandler.AddProofStep(lowerLevelProofSteps, standardizedR, qDelta,
composed);
thisLevelProofSteps.AddRange(lowerLevelProofSteps);
}
}
// return answers
return(thisLevelProofSteps);
}
